Evolution and roles of mesoscale convective system during the Nocturnal Rainfall in Sichuan Basin and its surrounding mountainous areas

The Sichuan Basin (SCB) and its surrounding mountainous areas have complicated topography, and the "Nocturnal Rainfall in the Basin (NRB)" and "Nocturnal Rainfall in the Mountainous Areas (NRMA)" are frequent. To further clarify the relationship and the interaction between the two types of "nocturnal rainfall" synoptic systems, the characteristics of the synoptic meteorology, thermodynamics, dynamics, and water vapor fields of the nocturnal precipitation process generated by two Mesoscale Convective Systems (MCSs) that originated from the mountainous area on the western side of the SCB on June 4, 2019 were analyzed and diagnosed in this paper. In addition, the WRF-LES model was used to simulate and analyze the macro and micro physical characteristics of two precipitation centers formed by the main system MCS1 in the center of the SCB and the mountainous areas around it. The results showed as follows. (1) Two MCSs originated from the mountainous area on the western side of the SCB were generated by the eastward movement of the low-pressure trough over the Tibetan Plateau (TP) coupling the higher Convective Available Potential Energy (CAPE) value with the unstable circulation of the upper level. They matured in the SCB and matured and split in the western end of Mt. Daba, respectively. (2) After splitting, the southern part of the sub-system MCS2 sank and moved south along the southern foot of Mt. Daba and uplifted the main system MCS1. After its explosive development, two precipitation centers with the characteristics of the NRB and NRMA were formed. (3) The vertical velocity, divergence, thermal helicity, and potential vorticity could be used as the thermodynamics and dynamics diagnostic quantities to indicate the occurrence and development of the two MCSs. The potential vorticity was an obvious precursory parameter compared with the other three. The water vapor flux divergence and moisture helicity could better indicate the vertical transport of water vapor in the systems. (4) The precipitation simulation result of the WRF-LES model on the main system MCS1 in the SCB was better than that on the sub-system MCS2 in the mountainous area on the northern margin of the SCB. In each MCS1 stage, the precipitation of the NRB was mainly induced by the cold cloud process (supplemented by the warm cloud process), while the precipitation of the NRMA was mainly induced by the warm cloud process. The combination of diagnostic analysis and numerical simulation could effectively promote an understanding of the relationship and interaction between the NRB and NRMA.